In order that an observer of a picture reproduction could not notice the structure of the printing raster, in black and white pictures the raster screen or screens, which are essential for reproduction, are rotated with respect to the horizontal. During the production of a multi-color picture the raster screen of the individual color recordings or components must be rotated even with respect to each other in order to avoid the so-called Moire effects or color play.
Such rotation of the raster presents no problem in connection with flat bed electro-type machines since with such machines it can be done by simply rotating the picture carrier and the recording carrier by the desired raster rotation angle with respect to the recording direction while the recording device (an engraving stylus, writing glimm lamp or some similar device) records the raster by the raster signals superimposed on the picture signals always along the direction of the relative movement between the table and the recording means.
The necessity sometimes arises that such pictures should be produced also by means of orthogonally oriented machines such as drum type scanners or electronic composing machines. With this last mentioned machines the difficulty arises to attain a proper rotation of the raster. An oblique securing of the picture set and of the recording carrier such as on the drum of a drum-type scanner, cannot be considered for several reasons, one of them, carrying the most weight, being that the advancement and the peripheral velocity of the drum are always rectangular with respect to the axis of the drum. The smallest deviations from the desired values would lead to unacceptable Moire formations.
Some attempts have been made in connection with drum type electro-type machines to remedy the above by an appropriate shifting of the raster points from line to line and by appropriately selecting their distances, whereby different preferred directions of the pattern were attained which corresponded to the rotated raster angles. In connection with these rasters which are only pretended to have a raster rotation however, in the reproduction there was always a certain Moire effect present.
Further it became known to scan a certain graphical raster picture together with the scanning of the picture and the raster signals so obtained to superimpose on the picture signals. This proposition can be actually used but requires the above-mentioned oblique positioning. There is a disadvantage, however, accompanying such process, in that additional scanning and a longer or sometimes an additional drum is necessary. As a result, the complexity of the apparatus is increasing while the rotation itself is not quite accurate and the above-mentioned disadvantages still remain unremedied. The possibility of obtaining a true raster rotation can be attained only by the digital technique.
According to known composing methods of producing half-tone raster pictures by means of a photo composing machine, the tone value can be sub-divided and numbered into an infinite number of grades between white and black according to a tone value scale. To each tone value there is a raster field assigned which contains a black point having different sizes, and called a spot. The size of this spot determines the tone value which the raster field on the location of the picture surface to which it belongs, represents. White and very bright picture portions are represented by small spots while dark or black picture portions are represented by large spots or such which merely covers the entire raster field surface.
Raster fields having black spots in them can be regarded under the concept of the known photo composing methods as a small picture the reproduction of which on corresponding apparatus produces electronic recording data which can be stored in a storage device.
In the graphic art raster fields containing spots therein irrespective of their size are called "raster points". In order to avoid any misunderstanding this expression here should be avoided and the expression "spot" or "raster spot" will be used throughout this application.
In order to record the raster pictures the recording data of the raster spots are called out from the storage device by the picture signals which have been obtained during the scanning of the picture. The picture signals control the either simultaneously performed recording or they are also stored for later use.
The above described methods are still unable to solve or offer a solution to the problem to perform accurate recording and reproduction when the raster structure is not orthogonal, that is, when it is obliquely oriented with respect to the horizontal of the field view of an observer or when several raster structures are printed on top of each other in a superimposed fashion and which are rotated with respect to each other.